docs: def get_grid_mapping_variables(ds):
      '''
      Returns a list of grid mapping variables
  
      :param netCDF4.Dataset ds: An open netCDF4 Dataset
      '''
      grid_mapping_variables = []
      for ncvar in ds.get_variables_by_attributes(grid_mapping=lambda x: x is not None):
          if ncvar.grid_mapping in ds.variables:
              grid_mapping_variables.append(ncvar.grid_mapping)
      return grid_mapping_variables